Ceramic hot-gas filter

ABSTRACT

A ceramic hot-gas candle filter having a porous support of filament-wound oxide ceramic yarn at least partially surrounded by a porous refractory oxide ceramic matrix, and a membrane layer on at least one surface thereof. The membrane layer may be on the outer surface, the inner surface, or both the outer and inner surface of the porous support. The membrane layer may be formed of an ordered arrangement of circularly wound, continuous filament oxide ceramic yarn, a ceramic filler material which is less permeable than the filament-wound support structure, or some combination of continuous filament and filler material. A particularly effective membrane layer features circularly wound filament with gaps intentionally placed between adjacent windings, and a filler material of ceramic particulates uniformly distributed throughout the gap region. The filter can withstand thermal cycling during backpulse cleaning and is resistant to chemical degradation at high temperatures.

This invention was made with Government support under Contract No.DE-AC21-94MC31214 awarded by the Department of Energy. The Governmenthas certain rights to this invention.

FIELD OF THE INVENTION

The present invention relates to a composite ceramic candle filter forremoving particulates from a hot gas stream, and a method for makingsaid filter.

DESCRIPTION OF RELATED ART

Ceramic filters have been tested in processes such as coal gasificationand coal combustion to remove particulates from hot flue gases toprotect downstream equipment from corrosion and erosion and to complywith EPA NSPS (New Source Performance Standards) regulations. Ceramicfilters in a tubular (candle) form, with one end closed and the otherend open have been shown to remove the particulates efficiently. The hotgas to be filtered typically flows from the outside to the inside of thefilter, with particulate-free gas exiting from the open end. The candlegeometry is also suited for removal of the filtered cake by backpulsingwith compressed gases.

Ceramic hot-gas candle filters must withstand exposure to chemicallycorrosive gas streams at temperatures in excess of 800° C. In addition,they are subjected to significant thermal stresses during backpulsecleaning which can cause catastrophic failure of the ceramic candlefilter element.

Ceramic hot-gas candle filters known in the art are generally fabricatedfrom either porous monolithic materials or porous ceramicfiber-containing composite materials. Monolithic ceramic candle filtersare either weak or can fail catastrophically in use. Composite filtersare less susceptible to catastrophic failure and generally have improvedstrength, toughness, and thermal shock resistance versus monolithicceramic filters.

Candle filters may have relatively uniform porosity throughout thefilter or they may comprise a porous support with a thin layer, ormembrane, of fine porosity on the outer surface of the support. Themembrane layer is typically applied to the filter using a variety ofmethods such as coating from a dispersion containing finer grains thanthose used in the support for smaller membrane pore sizes, bondingrandomly arranged chopped ceramic fibers to the support using colloidal(or sol) materials, or forming a ceramic matrix by chemical vaporinfiltration.

Materials used to fabricate ceramic hot-gas filters generally includeoxides such as aluminosilicates, glass, and alumina, and non-oxides suchas silicon carbide and silicon nitride. Oxide-based ceramic filters haveadequate resistance to flue gas atmospheres and fly-ash for the designlife of the filters; however, they generally have low thermal shockresistance. Non-oxide ceramics generally have good thermal shockresistance, however they are susceptible to oxidation in the corrosiveenvironment to which they are subjected which results in a degradationof mechanical properties.

The disadvantages of ceramic candle filters known in the art includefailure, often catastrophic, due to thermally induced stresses caused bybackpulse cleaning, chemical degradation caused by species present inthe hot gases being filtered, delamination of the membrane layer,incomplete removal of the filter cake upon backpulsing, and high cost.They also tend to be heavy, requiring expensive support structures tohold an array of the candles in the filter unit.

SUMMARY OF THE INVENTION

The present invention is directed to a ceramic hot gas filter comprisinga porous elongated filter support and a porous membrane layer on atleast one surface thereof. Specifically, the porous membrane may be onthe outer surface, the inner surface, or both the outer and innersurface of the porous elongated filter support. The membrane layer(s) isfirmly adherent to the support and therefore does not suffer fromdelamination problems. The porosities of the support and membrane arecontrolled such that the support functions as a bulk filter and themembrane layer functions as a surface filter. The support has an openingat one end into a hollow interior, a closed end opposite the open end,and an external flange integral with the open end. The support is formedof a plurality of layers of oxide ceramic support yarn, each layer beingarranged in a crisscrossing relationship with neighboring layers to forma plurality of quadrilateral-shaped openings. The yarn in the support iscoated with a first oxide ceramic material which, upon heat treatment,forms a porous refractory oxide support matrix. The membrane layer(s)may be formed of an ordered arrangement of continuous filament oxideceramic membrane yarn, a uniform coating of ceramic filler material, orsome combination of the two. Any yarn present in the membrane layer is(preferably prior to winding) coated with a second oxide ceramicmaterial which, upon heat treatment forms a porous refractory oxidemembrane matrix. Preferably, the support yarn and the continuousfilament membrane yarn each contain at least 20 weight percent alumina(Al₂ O₃) and have softening points above about 750° C. The ceramiccoating materials are generally particulates of oxides or oxidecompounds, or mixtures thereof and may also include oxide precursormaterials. The membrane layer(s) has a porosity that is less than thatof the support. Preferably the quadrilateral-shaped openings havedimensions of about 100 to about 500 microns after heat treatment sothat the support functions as a bulk filter. The membrane layer(s)preferably has pore diameters of about 0.1 to 50 microns and functionsas a surface filter. In a preferred embodiment of the invention, thesupport yarn has generally the same composition as the membrane yarn andthe support matrix has generally the same composition as the membranematrix.

The present invention also provides a method for making a ceramic hotgas filter involving the steps of fabricating an elongated porous filtersupport by coating a ceramic oxide support yarn with a first coatingcomposition, winding the coated support yarn onto a mandrel to form aplurality of layers of the coated support yarn, each layer beingarranged in a crisscrossing relationship with neighboring layers to forma plurality of quadrilateral-shaped openings. The mandrel may becontoured to provide an integral external flange adjacent to one end ofthe support. Alternatively, a separate collar insert may be slid onto auniformly cylindrical mandrel to form the flange portion of the support.The resulting support has an open end adjacent to the flange, an outsidesurface, and a second open end opposite the flanged end.

A membrane layer is then formed on at least one surface of the support.For example, the membrane layer may be formed on the outer surface bycoating a continuous filament oxide ceramic membrane yarn with a secondcoating composition and applying the coated membrane yarn in an orderedarrangement on the outer surface of the support. Methods for forming theordered arrangement membrane layer(s) include hoop winding a singleyarn, multiple yarn winding, fabric wrapping and coating with aparticulate slurry or a solution containing ceramic precursor materials.In a preferred embodiment, the ordered arrangement comprises a circularor hoop winding of the continuous filament oxide ceramic so as to definea gap of predetermined width between adjacent windings. The gap is thenfilled with additional ceramic filler material, preferably an oxidematerial, which upon subsequent heat treatment, forms a porousrefractory membrane matrix. The width and uniformity of the gap betweenadjacent hoops or windings is not particularly critical; however,uniform filling of the gap with filler material is desirable, botharound the circumference and along the length of the filter. In anotherembodiment, a membrane layer is formed by winding the coated continuousfilament such that adjacent hoops or windings are as close to oneanother as possible and no such filler material is applied. Yet anotherembodiment features a membrane layer comprising ceramic filler materialbut without hoop-wound filaments; i.e., an infinitely large gap betweenhoop windings. In this embodiment, for example, ceramic particulates,preferably of an oxide material and preferably in the form of a slurry,are applied to the support layer as uniformly as possible, so as toessentially close off the diamond shaped openings formed by thecrisscrossing filaments of the support layer. A slurry is a convenientform for the filler material because the slurry is amenable to beingpainted by brush or spray, or being dip coated, etc. Another useful formfor providing the ceramic filler material to the developing candlefilter is as a paste, which may then be applied using, for example, aspatula-like flexible applicator. Other media for communicating thefiller material to the developing candle filter may occur to an artisanof ordinary skill and should be considered therefore to be within thescope of the present invention.

Once the support layer has been wound, the support mandrel removed, andthe membrane layer(s) formed, the second open end (opposite the flangeend) is closed using an oxide ceramic material. The support and membranelayer(s) are heat treated to convert the first coating composition to aporous refractory oxide support matrix and to convert the variouscoating compositions to a porous refractory oxide membrane matrix.

The present invention provides a strong, lightweight ceramic hot-gascandle filter which has a greater than 99.5% particulate collectionefficiency, thus meeting EPA NSPS regulations. Failure of the filter isgenerally not catastrophic since if the membrane is damaged, the supportquickly blinds at the location of the damage due to its bulk filtrationproperties, thus preventing release of particulates and protectingdownstream process equipment such as gas turbines or sorbent beds. Thefilter of the present invention is resistant to chemical degradation dueto the oxide compositions used, and at the same time provides excellentthermal shock resistance which is not generally typical of oxidematerials. The smoothness of the membrane surface(s) results inefficient removal of the filter cake during backpulse cleaning. Inaddition to the above-mentioned advantages, the filter of the currentinvention is potentially low cost relative to most of the commerciallyavailable candle filters.

DEFINITIONS

"Ceramic" as used herein means crystalline or partially crystallinematerials, or non-crystalline glasses, which comprise essentiallyinorganic, nonmetallic substances.

"Continuous fiber or filament" as used herein means a fiber or filamenthaving a length which is at least 1000 times the diameter of such fiberor filament.

"Filler material" or "membrane filler material" as used herein meansthose bodies in the membrane layer other than those bodies making up theyarn or any slurry material coated on the yarn. As such, the fillermaterial may be in the form of powders, particulates, whiskers, choppedfibers, platelets, flakes, spheres, tubules, pellets, etc.

"Membrane" or "membrane layer" as used herein refers to that layer whichis deposited or applied onto at least one surface of the support layer,has a lower porosity than the support layer, and which provides themajority of the filtering action.

"Oxide" as used herein is meant to include oxides, oxide compounds (e.g.mullite, spinel), or precursors thereof.

"Support" or "support layer" as used herein refers to the structureformed by winding single or multiple continuous ceramic fiber orfilament around a mandrel in a crisscrossing arrangement to produce anordered array of diamond shaped openings. The function of the support orsupport layer is to provide a suitably strong foundation to which themembrane adheres.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of an embodiment of a filterelement of the current invention, including an optional flange collarsection.

FIG. 1B is a cross section of the filter element taken on line 1B--1B ofFIG. 1A.

FIG. 1C is a cross section of the flange section taken on line 1C--1C ofFIG. 1A.

FIG. 1D is a cross section of the flange section taken on line 1D--1D ofFIG. 1A.

FIG. 1E is a cross section of the closed end taken on line 1E--1E ofFIG. 1A.

FIG. 2 shows openings formed by the overlap of two layers of yarn in asupport layer comprising an embodiment of the present invention.

FIGS. 3A, 3B, and 3C are cross-sectional views of the filter wall whichillustrate the construction variations of the membrane layer.

DETAILED DESCRIPTION OF THE INVENTION

The hot-gas filter of the current invention is of the candle filter typeand comprises a porous ceramic support having a porous ceramic membranelayer on at least one surface thereof. Specifically, the porous membranemay be on the outer surface, the inner surface, or both the outersurface and inner surface of the porous ceramic support. The membrane isless porous than the support and serves as a surface filter, preventingpollutant particulates from penetrating therethrough. The support hasgood filtration capacity for fly-ash and serves as a bulk filter,capable of trapping particulates between its inner and outer surface,should membrane leakage occur.

Referring to FIGS. 1A-1E, the filter 10 comprises a support 12 having agenerally elongated tubular shape with an open end 14 at one end into ahollow interior. The end 15 of the support opposite the open end isgenerally closed. The support further includes an external flange 16integral with the open end 14 which supports the filter in a tube sheetin use. The flange may also include an optional collar insert 24,integral with the flange, and described in more detail below. Themembrane layer(s) 18, 23 are formed on the outer surface 20 of thesupport and/or the inner surface 22 of the support. End 15 is generallyclosed by filling with a ceramic material 26, and the flange section 16and tip section of the support adjacent the closed end 15 are madeimpervious as described below.

The overall porosity of the support layer is determined by a combinationof the open volume created by the diamond or parallelogram-shapedopenings (macropores) and the porosity of the matrix coating surroundingthe individual yarns (micropores). The porosity of the membrane layer isdue primarily to the porosity between adjacent particles making up thelayer (micropores) or due to microcracks.

The macroporosity of the support may be calculated from the volume ofthe support (calculated from the measured dimensions of the support),the weight of the support, and the bulk density of the support (fiberand matrix, including any microporosity). The bulk density is measuredusing mercury porosimetry.

The matrix is applied in such a way that the channels in the support arenot substantially closed. The matrix generally imparts integrity andmechanical strength to the support and also provides an excellent degreeof thermal shock resistance because of the ability of the porous matrixto absorb thermally induced mechanical stresses which might otherwisefracture the fibers in the filter.

The support is formed of a plurality of layers of continuous ceramicoxide yarns which are laid down in spaced helical coils in acrisscrossing relationship with neighboring layers to form a pluralityof diamond or quadrilateral-shaped openings having dimensions between100 and 500 microns after firing. The openings form channels extendingbetween the inner 22 and outer 20 surfaces of the support which followtortuous, curved paths (see FIG. 1B). If the filter is damaged, forexample by damaging the membrane layer during installation, it willquickly "self-heal" by functioning as a bulk filter and blinding withparticulates in the hot gas stream. A support containing a significantnumber of straight radial channels will not blind as readily, resultingin failure of the filter. Forsythe, U.S. Pat. No. 5,192,597,incorporated herein by reference, describes filament winding ofreticulated ceramic tubes in a preferred winding pattern. The yarns inadjacent layers of diamond-like patterns are laid down in such a mannerthat the yarns forming the walls of the diamonds of each layersubstantially cover the diamond shaped openings of each adjacent layer.This forms a tubular structure comprising series of interconnecteddiamond shaped openings, each layer of which interfere with the directflow of gas from one layer to the next.

The winding pattern described above is for the elongated central bodysection of the support (i.e. the generally cylindrical section of thefilter between the flange and closed end). Due to the contoured closedend and flange sections of the filter, the described winding pattern isnot achieved at the flange and closed end.

FIG. 2 shows two adjacent layers of yarn in a support prepared accordingto U.S. Pat. No. 5,192,597 (the matrix layer is not shown in thisFigure) which define openings designated by "x". The size of theopenings is controlled by the spacing between the yarns in each layerwhich is determined by the wind angle and yarn denier in addition to theamount of matrix material applied to the yarn. The spacing "a" betweenadjacent yarns is preferably controlled to provide openings havingdimensions "a" of between about 100 and 500 microns in the finalsupport, after high temperature firing. The openings have more of asquare shape near the inner surface of the support, with one of thediagonals gradually increasing in size, as winding continues, to theouter surface, thereby according the opening more of a diamond shape.The dimension "a" can be calculated based on the yarn spacing and theamount of matrix applied to the yarn. Alternately, "a" can be measuredvisually in the final support. A support having the describedconstruction and having openings in this size range will function as abulk filter which can trap particulates within the wall of the supportwhile maintaining a pressure drop that is insignificant relative to thepressure drop across the membrane layer.

The support may be formed by winding a ceramic oxide yarn on a suitablydesigned mandrel using a filament winder designed to maintain a constantwinding ratio (rotational speed of the mandrel divided by the speed ofthe traverse arm). A constant winding ratio is necessary to maintain theproper size and distribution of channels throughout the wall. The flangesection of the support is formed by using a mandrel that is wider at oneend, the wide end being contoured to give the desired flange geometry.Filament winding on such a mandrel produces a tube with an externalflange section at the open end and a small hole at the opposite end,which is generally closed in the final support with a ceramic material26 as shown in FIG. 1E. Alternately, if it is desired that the insidewall of the support be straight as opposed to contoured at the flangesection, a filament wound collar insert 24, shown in FIG. 1C and FIG.1D, having a composition similar to that of the support and having aninner diameter approximately equal to the outer diameter of the mandreland an outer surface contoured to give the appropriate flange geometrymay be used. The collar is then placed on the mandrel and the support isthen wound on the combined mandrel and collar. When the support isremoved from the mandrel, at least a portion of the collar remains withthe support as part of the flange section, as illustrated in Example 2below.

Field tests have demonstrated that hot-gas candle filters commonly failat the flange section. According to the current invention, the flangeand the body of the support are formed as a single unit to ensurehomogeneity of the support material across the entire filter and toeliminate any stresses or weak spots arising from joining materials. Theshape of the flange is not critical but should be reproducible. Theflange should provide a good seal with the tubesheet that supports thefilter in use so that no dust leakage occurs. The shape of the closedend is generally round, but various shapes are possible by suitablyshaping the mandrel. The diameter of the opening at the closed end ofthe tube depends on the diameter of the shaft that supports the mandrel.

The membrane layer is applied to the outer surface of the support, theinner surface of the support, or to both the outer and inner surface ofthe support. The membrane layer usually comprises an ordered arrangementof continuous filament oxide ceramic yarns. The membrane layeroptionally may also comprise one or more ceramic filler materials tohelp fill gaps or plug cracks, voids, etc. between adjacent yarns. Inthe alternative, the membrane layer may comprise the ceramic fillermaterial, but no ceramic yarns. The membrane layer(s) in the finalfilter, after heat treatment, has pore diameters of between about 0.1-50microns, preferably 5-25 microns. Preferably, the average pore size andsize distribution is substantially invariant around the circumferenceand along the length of the filter.

For those embodiments in which the membrane layer(s) comprises yarns,the ordered arrangement of yarns in the membrane layer(s) may be formedby various methods including circular (hoop) winding, multiple yarnwinding, or wrapping with yarns prearranged in two or three dimensionalforms such as fabric or braided materials. The membrane yarns should bearranged so as to obtain a smooth membrane surface. A smooth membranesurface is desirable because it facilitates complete removal of thefiltered material during backpulse cleaning because the filter cakereadily debonds from the smooth surface. If the surface is rough, thefiltered cake tends to be mechanically anchored to the surface making itdifficult to completely remove the cake by backpulse cleaning. Thecircular winding produces a smooth membrane surface.

The yarns used to form the support and membrane layer(s) preferablycomprise ceramic fibers having softening points of at least about 750°C., more preferably at least 1000° C. The phrase "softening point" isused herein to mean both the softening point of a glass ceramic and themelting point of a crystalline ceramic. The yarns used in the membranelayer(s) may be the same as or different than the yarns used in thesupport.

Suitable oxide fibers include, for example, certain glass fibers such asS glass (high tensile strength glass containing about 24-26% alumina(Al₂O₃)), "Fiber Frax" alumina-silicate fiber, and polycrystallinerefractory oxide fibers containing at least about 20% by weight ofalumina such as the alumina-silica fibers disclosed in U.S. Pat. No.3,503,765 to Blaze and certain of the high alumina content fibersdisclosed in U.S. Pat. No. 3,808,015 to Seufert and U.S. Pat. No.3,853,688 to D'Ambrosio. Preferably the oxide fibers comprise between20% and 80% by weight of aluminum oxide. Examples of commerciallyavailable aluminosilicate fibers include "Altex" (Sumitomo) and "Nextel"(3M) fibers. Fibers containing significant levels of glass-formingoxides such as B₂ O₃ and P₂ O₅ are not desirable because they will fluxthe entire structure resulting in a dense, nonporous support.

Fibers of refractory oxide precursors can also be used to form thesupport. After winding, the precursor fibers are converted topolycrystalline refractory oxide fibers by firing to remove volatiles,convert salts to oxides, and crystallize the fiber. The preparation ofrefractory oxide fibers and their precursors is disclosed in U.S. Pat.Nos. 3,808,015 and 3,853,688.

The oxide fibers generally have diameters in the range of 0.2 to 2.0mils (0.005-0.05 mm) and are used in the form of continuous yarns,preferably containing 10-2,000 or more fibers. The fibers are preferablycontinuous filaments, however yarns of staple fibers can be used,especially glass. The yarns are preferably loosely twisted so that anyloose or broken ends do not interfere during filament winding when theyarn is pulled through small orifices. The yarns may also be used in theform of roving. Bulked, interlaced, or textured yarns may be used.However, the yarns used in the membrane layer most preferably comprisecontinuous filament, untextured yarns so as to obtain a membrane layerhaving a smooth outer surface. Glass yarns which crystallize to formrefractory oxides upon high-temperature heat treatment are preferredbecause they are easier to handle and less likely to break duringfilament winding than yarns containing crystalline ceramic fibers.

The refractory oxide matrix components of the support and membranepreferably have softening points above 1000° C., more preferably aboveabout 1400° C., and most preferably above 2000° C. Preferably the matrixcomprises at least 40 wt % alumina.

The matrix components are generally applied to the support and membraneyarn(s) in the form of a coating composition which is then fired to forma refractory oxide matrix. The coating composition used in the supportmay be the same as or different than the coating composition used in themembrane. The coating composition generally comprises an aqueoussolution, suspension, dispersion, slurry, emulsion, or the like whichcontains one or more oxide particulates or oxide precursors. Preferablythe oxide particulates have a particle size of 1-20 microns, morepreferably 1-10 microns, most preferably between 1-5 microns. Particlesizes less than 20 microns are preferred because they are readilydispersed and penetrate into voids between fibers. Slurries preparedusing particle sizes less than 1 micron are generally too viscous atuseful solid concentrations. Oxide particulates useful as matrixmaterials include alumina, zirconia, magnesia, mullite, spinel, etc.Suitable matrix precursors include water soluble salts of aluminum,magnesium, zirconium and calcium such as "Chlorhydrol®" (aluminumchlorohydrate solution sold by Reheis Chemical Co.), zirconyl acetate,alumina hydrate, basic aluminum chloracetate, aluminum chloride, andmagnesium acetate.

Preferably, drying control additives such as glycerol and formamide maybe added to the coating composition at levels of 1-5 wt % based on thetotal weight of the coating composition. The drying control additivesreduce drying stresses in the green body and also eliminate macroscopiccracks on the surface of the high-temperature fired filter. Moreover,drying stresses can be further reduced by winding the support andmembrane layer(s) in an environment with a relative humidity of at leastabout 30%.

The coating composition preferably includes a ceramic oxide precursor toincrease the green strength of the wound structure. These soluble oxideprecursors which are useful as matrix precursors also function asbinders. A preferred binder is aluminum chlorhydrate, and in particular,the above-mentioned "Chlorhydrol". Preferably the coating compositionincludes between about 10-25 wt % binder, calculated based on the totalsolids content of the coating composition. The aluminum chlorohydrateserves to bond the oxide particulates of the coating together andincreases the green strength of the support. The binders areincorporated into the refractory matrix upon heat treatment.

The coating composition may be applied to the support by drawing theceramic oxide yarn through the coating composition prior to winding on amandrel. Preferably, the coating composition is uniformly distributedaround the fibers of the yarn. The distribution is affected by theviscosity of the coating composition, the method of application, thedensity (or tightness) of the yarn bundle, the nature of the yarn andthe amount of the coating composition. The composition should have aviscosity that is low enough to permit flow and some penetration intovoids in the yarn but high enough to adhere to the yarn bundle. When thecoating composition is a particulate slurry, the solids content ispreferably between 50-75 wt % and the slurry preferably has a viscosityin the range of 100-300 centipoise. If a coating composition containingboth an oxide precursor and particulate oxide powder is used, the solidscontent of the slurry should be adjusted to about 60-90 wt % of therefractory oxide matrix material derived from the oxide particulate andabout 10-40 wt % derived from the precursor. It is difficult to obtainsufficient amounts of oxide-containing materials in the coatingcomposition using levels of precursor greater than about 40 wt %. Theamount of matrix material applied to the yarn can be controlled bypulling the yarn through a suitably sized orifice to remove excessslurry. The coating composition may be also be applied to the yarn byuse of a finish roll, spraying, etc. Further, the matrix coatingcomposition may be applied to the wound filamentary membrane and supportby dipping the wound support in a slurry, draining off the excess anddrying. Additional dipping steps may be used if necessary to provide thedesired weight of matrix relative to the weight of yarn in the support.In general, it is difficult to apply the matrix coating composition bydipping without closing a significant portion of the channels in thesupport, which is not desirable and results in increased backpressure.

The membrane matrix coating composition may be applied to the membraneyarn using methods similar to those described for the support.Preferably the combined weight of the matrix components of the supportand membrane layers comprises about 40-70% of the final weight of thefilter, more preferably about 50-60%. To avoid thermal stresses, it ispreferable that the support yarn has generally the same composition asany membrane yarn and the support matrix has generally the samecomposition as the membrane matrix. In certain applications, however,different compositions may be desirable. For the same reason, it ispreferable to have a weight ratio of fiber to matrix which isessentially the same in the membrane and the support.

In one embodiment, multiple yarns are combined and wound on the supportat substantially the same wind angle as that of the support to fill theunderlying (or the eventual overlaying) openings in the support. Thismay be accomplished by feeding the separate yarns through tensioningdevices, dipping in a ceramic matrix particulate slurry, and combiningthe yarns just prior to pulling through a larger sizing orifice thanthat used for single yarn ends and winding on the support if an outersurface membrane is desired, and on the mandrel if an inner surfacemembrane is desired. The diameter of the sizing orifice is selected asdescribed above for hoop winding.

A membrane comprising a single filament wound layer on the support or onthe inner surface of the support is generally adequate for manyfiltration applications. Additional layers of wound yarns may be appliedto increase the thickness of the membrane layer. This usually increasesthe particulate collection efficiency and the back pressure of thefilters.

In another embodiment, the membrane layer is formed by wrapping thesupport or mandrel with a ceramic fabric. The fabric is wrapped on thefilter support or mandrel and a matrix slurry similar in composition tothat used in the support is brushed on the fabric. The slurry wets thefabric and the support, and provides bonding to the support. Anywrinkles in the fabric are removed while still wet. Additional layers offabric are wrapped on the support or mandrel as necessary to increasethe filtration efficiency. The fabrics useful for building the membranelayer include tightly woven plain and satin weaves. It may be necessaryto use a matrix slurry containing matrix particulates having a smallerparticle size than the matrix particulates used to wind the support inorder to improve the adhesion between the filter support and fabricmembrane layer. This is because the smaller particles will more readilyinfiltrate the interstices in the woven fabric. In general, this methodis less preferred because it is more difficult to control the amount ofmatrix applied to the membrane layer. In addition, it has been foundthat the fabric layers tend to be less strongly adhered to the supportthan membranes formed using the filament winding techniques describedabove.

In still another embodiment, the membrane is formed by hoop winding. Theoxide ceramic membrane yarn is coated with the membrane matrix coatingcomposition, for example by passing through a bath containing a coatingcomposition, followed by passing through a sizing orifice to removeexcess slurry, and winding at approximately 90 degrees to the axis ofthe mandrel. Preferably, the diameter of the sizing orifice is carefullyselected to give a matrix pick-up that yields similar weight ratios offiber and matrix in the membrane and support layers. The rate of mandrelrotation relative to the rate of the movement of the transverse armcontrols the spacing between adjacent yarns. FIG. 3A illustrates thecross-sectional view of a filter created in this fashion, where "a"represents the end view of a yarn in the support body, "b" representsthe end view of a yarn in the hoop-wound membrane, and "c" representsthe spacing between adjacent yarns in the hoop-wound membrane. In oneversion of this embodiment, the slurry coated membrane fiber or yarn iswound as close as possible with substantially no overlapping of yarns orintentional gaps between yarns in the membrane layer (e.g., dimension"c" in FIG. 3A equals zero). Filtering action is provided by themicro-cracks in the matrix material between adjacent yarns. Optionally,filler material may be applied to the wound membrane to fill in anyunintentional gaps between adjacent yarn hoops.

In another version of this embodiment, an intentional gap is leftbetween adjacent hoop windings of the slurry coated yarn. Additionalceramic filler material, (e.g., particulates) or a precursor to aceramic filler material, preferably in the form of a slurry, is thendeposited in this gap. A membrane formed in this way is termed a"combination membrane". Preferably the gap-filling slurry should containa suspension agent to maintain a uniform consistency. The desirableviscosity will depend on the method of application chosen; lowviscosities are best suited for a brushing technique, paste-like highviscosities are more appropriate when applying with a spatula. FIG. 3Billustrates the cross-sectional view of a filter having a combinationmembrane, where "a" represents the end view of a yarn in the supportbody, "b" represents the end view of a yarn in the hoop-wound membrane,"c" represents the spacing between adjacent yarns in the hoop-woundmembrane, and "d" represents the filler material used to fill the spacesbetween adjacent yarns. This additional ceramic filler material may beof the same chemical composition as the membrane matrix material coatingthe membrane yarn, or it may have a different chemical composition.Typically, constituents used for gap filling are larger (e.g., 25-75microns) than those particulates used for matrix formation (e.g., 3-5microns). Further, the intentional spacing "c" is almost infinitelyvariable; it may range from substantially zero to many times thediameter of a yarn.

In yet another embodiment, because there appears to be no upper limit tothe size of the gap between adjacent windings in the membrane layer, itis possible to dispense completely with the hoop wound slurry coatedyarn, leaving the membrane layer to consist essentially of the ceramicfiller material. FIG. 3C illustrates the cross-sectional view of afilter created in this fashion, where "a" represents the end view of ayarn in the support body and "d" represents the ceramic filler materialused to fill the quadrilateral-shaped openings at the surface of thesupport body. Again, such material preferably is in the form of a slurryor solution which can be applied directly to the support layer bybrushing, spraying, dip coating, etc. Also, the preferred size of theconstituents making up a "filler material only" membrane layer is about25-75 microns in diameter.

The above discussion generally pertains to the form of the invention inwhich the membrane layer is applied to the outer surface of the supportlayer. When the membrane layer is to be applied to the interior surfaceof the support layer, the fabrication procedures may have to bemodified. For example, when a fiber or fabric is to make up the membranelayer, it may be preferred to wind or wrap such fiber or fabric over themandrel before winding the support layer. Also, if the inner membranelayer is "filler material only" or if an additional slurry or solutionis to be deposited onto a hoop-wound filament layer, it may be preferredto do so once the support layer has been formed and the mandrel has beenremoved. Further, it may be impractical to apply a slurry or solution tothe interior of a tube by brushing or spraying. In such a case, slipcasting or drain casting should achieve the desired results.

The flange section and the closed end may be reinforced and madeimpervious to any gas streams by saturating with additional ceramicslurry or using a ceramic cement composition. To avoid reactions withthe underlying support material and to match the thermal expansion ofthe support, the matrix material used in the support is preferred forthis purpose. After winding and reinforcing one or both ends, the candlefilter is dried at room temperature while on the mandrel until it isstrong enough to handle.

After overnight drying (about 12-16 hours) at ambient temperature, theends of the developing filter are cut off so that the mandrel may beremoved. Specifically, the collar portion of the filter is sliced suchthat a section of the original collar remain in the flange section ofthe support layer (see FIG. 1D).

The developing candle filter may then be fired at temperatures below thesoftening point of the ceramic yarn and sufficiently above the boilingpoint of any volatiles, typically around 300° C. to 800° C., to removethe volatiles and stabilize the filter. This is especially importantwhen oxide precursors are used.

Closing off the tip may then be accomplished using commercial hightemperature cements or by filling with a high viscosity paste (similarin composition to the matrix coating slurry) mixed with a small amountof the type of yarn used in the support structure, or by filling withthickened paste similar to the membrane filler material. The solids incommercial cement should not react with the tube material to reduce thethermal stability of the filter. It is also preferable to have fired thecandle filter, as described above, prior to the application of a ceramicfiller material to the membrane layer(s).

An additional firing at high temperatures is then carried out, typicallyat 1200° to 1400° C., to form stable crystalline phases. Firing above1450° C. may melt some of the phases and result in a fused product whichis undesirable due to reduced thermo-mechanical properties. Preferably,the heating rate during the high temperature firing does not exceed 20°C. per minute, in order to allow any glass phases to crystallize, andmay be as low as 0.1° C. per minute. During high temperature firingglass fibers may devitrify into crystalline phases, the matrix mayconvert to stable crystalline phases or the crystalline phases in thefiber and matrix may react to form new stable crystalline phases. Thefinal phase composition of the product depends on the amounts of fiberand matrix, the heating profile, soaking time at intermediatetemperatures and the dwell time at the highest firing temperature. Thetypical crystalline phases are corundum, mullite, cordierite andcristobalite. As used herein, the term cordierite is intended to includeindialite, a crystalline material having the same composition ascordierite, but a slightly disordered crystal structure. Excesscristobalite formation is undesirable since cristobalite undergoes avolume change at 200-270° C., which contributes to poor thermal shockresistance. The final filter should contain no more than 10% by weightcristobalite. Preferably the final composition of the filter is 3-7parts by weight magnesia, 20-45 parts silica and 45-70 parts alumina.More preferably the final filter comprises between about 60%-70%alumina.

In a preferred embodiment, the yarn used to prepare both the support andmembrane comprises glass fibers comprising 61-66% SiO₂, 24-26% Al₂ O₃,and 9-10% MgO. A coating composition consisting essentially of aluminais applied to the yarn prior to winding in an amount sufficient toprovide a refractory oxide matrix comprising 40-70% of the final weightof the filter. The coating composition contains a binder comprisingaluminum chlorhydrate and alumina matrix particulates having an averageparticle size of 2-3 microns. The membrane is applied to the support ormandrel by hoop winding. The as-wound filter element is heated to removevolatiles and then high temperature fired at temperatures above about1350° C., preferably at a temperature of about 1380° C. During hightemperature firing, the glass fiber softens and a portion of the silicaand magnesia in the glass combine with the alumina matrix material toform cordierite and mullite. The final filter comprises about 20-40% byweight SiO₂, about 3-6% by weight MgO and about 50-70% by weight Al₂ O₃.The final crystalline composition, after heat treatment, is 25-40%cordierite, 5-15% mullite, 40-60% corundum and 0-10% cristobalite, basedon the total crystalline content. Approximately 50-90 vol % of thematerial is crystalline with the remainder being amorphous. Theformation of crystals of mullite, cordierite, and corundum, each havingdifferent coefficients of thermal expansion, leads to formation ofmicrocracking in the structure. The microcracks form along crystallineboundaries as well as within regions having only a single crystal phase.The microcracks are believed to absorb stresses caused by thermal shock.After firing, the filter is stable up to 1200° C. for extended periodsof time and has excellent thermal shock resistance.

EXAMPLES

All percentages referred to herein are weight percent, unless otherwiseindicated.

The filament winder used to wind the support in the Examples below had achain-driven traverse of approximately 70 inches (178 cm) (278 teeth of0.5 inch (1.27 cm) pitch passing in a narrow loop driven and supportedby 11 tooth drive sprockets at each end). The drive ratio was set suchthat the spindle rotated at a speed of 50 and 10/111 revolutions foreach complete rotation of the chain loop for winding of the filtersupport. The mandrel was a tube having a length of 65 inches (165 cm)and an outer diameter of 1.75 inches (4.45 cm) with end closures at eachend. One of the end closures was conical with about a 30 degree taper oneach side of the cone with a 0.50 inch (1.27 cm) diameter drive shaftmounted at its axis. The second end closure was hemispherical (1.75 in(4.45 cm) diameter) with a 0.25 inch (0.64 cm) drive shaft mounted atits axis. The mandrel was attached to and driven by the spindle in sucha position as to be traversed along its length by the traversing yarnguide. The mandrel was attached to and driven by the spindle via the0.50 inch (1.27 cm) shaft and supported in a bearing at the 0.25 inch(0.64 cm) shaft. It was mounted parallel to the chain-driven traverseguide such that the guide traversed above the mandrel surface at adistance of about 0.75 inch (1.91 cm) from the surface of the mandreland the traverse stroke extended from about 0.75 inch (1.91 cm) past thehemispherical closure onto the 0.25 inch (0.64 cm) shaft and to about0.75 inch (1.91 cm) past the conical closure onto the 0.5 inch (1.27 cm)shaft.

A plastic collar having a 7 mm wall thickness and a 45 degree edgerelative to the axis of the collar was inserted on the mandrel near theconical end to form the flange on the filter support for Examples 1 and3.

For Example 2, a separate winder having a 6 inch (15.2 cm) traversestroke with means to adjust this stroke to contour the package ends wasused to form a collar insert for the flange section of the filter. Thedrive ratio was set such that the spindle rotated at a speed of 4 and11/180 revolutions for each complete rotation of the traverse cam toprovide the same wind angle in the collar insert as the wind angle inthe support. A mandrel comprising a short piece of 1.75 inch (4.45 cm)outer diameter tube was mounted on the spindle and wrapped with 2 layersof 0.002 inch (0.005 cm) thick "Mylar" polyester film to facilitateremoval of the wound unit. The mandrel was wrapped with 90 grams ofS-glass (S-2 CG150 1/2 636, available from Owens-Corning FiberglasCorporation of Toledo, Ohio) that was coated with an aqueous A-17alumina slurry (see Example 1 for composition of slurry) applied in sucha quantity to form a unit having 50-60 wt % ceramic from the slurry and40-50 wt % ceramic derived from the feed yarn after drying. The collarinsert, as wound, had the form of a cylinder of approximately 1.75 inch(4.45 cm) inner diameter and a 3/8 inch (0.95 cm) wall thickness withthe ends of the cylinder wall exhibiting a taper of approximately 45°.The insert was removed from its mandrel while still wet and transferredto the mandrel on the main filament winder, described above. The insertwas positioned so as to leave about 57 inches (145 cm) of the straighttube portion of the mandrel exposed between the insert edge and thejunction of the tube with the hemispherical end closure.

The filter support units were wound onto the mandrels with either thecollar insert or plastic collar mounted thereon. Winding was carried outwith the spindle set at a rotational speed of approximately 500-520revolutions per minute. The final (fired) support units haddiamond-shaped openings on the outer surface having dimensions of about175-250 microns.

TEST METHODS

The density and porosity of the membrane layers was determined usingmercury porosimetry. Membrane samples were prepared for porosimetrymeasurements using either of two methods. The membrane layer can bereadily debonded from the support prior to firing of the candleassembly. The debonded membrane layer is then high-temperature fired andsubmitted for porosimetry measurements. Alternatively, the membranesample may be prepared by scraping away the support layer from a sampleof a high-temperature fired candle assembly. The median pore size isreported in microns and the porosity is reported in volume percent. Themedian pore size is the value obtained at the maximum intrusion volume.

The average oxide composition was determined using X-ray Fluorescencespectroscopy. The samples and standards were fused in a lithiumtetraborate flux and the X-ray emission lines for the elements ofinterest were measured. The results are reported as weight percent withthe samples being dried at 130° C.

Crystalline phase compositions were determined using X-ray diffractionusing a Scintag Pad X theta-theta diffractometer using Cu K-alpharadiation. The following conditions were used: copper tube operated at45 kilovolts, 40 milliamps, goniometer radius 250 mm, beam divergence0.24 degree, scatter slit 0.43 degrees, receiving slit 0.2 mm, germaniumsolid state detector bias 1000V, scan speed 0.2 degrees 2-theta perminute, chopper increment 0.03 degrees 2-theta, scan range 3 to 112degrees 2-theta (overnight scans), samples front packed against filterpaper in a 1 inch square aluminum well-type sample holder, single samplechanger. The samples were wet milled in acetone for 5 minutes in aMcCrone vibratory mill using corundum grinding elements and dried undera heat lamp. The percentages of crystalline phases were determined basedon a mixture of standard materials with 20% fluorite as an internalstandard. Standard materials used were NIST (NBS) 674 alpha alumina(corundum), Baikowski high purity cordierite (indialite) standard, Coorsmullite standard, NIST (NBS) 1879 cristobalite, NIST (NBS) 1878 quartz,and Coors spinel standards. The samples themselves were not mixed withan internal standard but were normalized to 100% of the crystallinecomponents after dividing each measured intensity by its respectivereference intensity ratios. Analysis lines were: indialite at 10.4, 18.2and 29.5 degrees; mullite at 16.5 and 26.1 degrees, corundum at 25.6 and52.6 degrees, cristobalite at 21.8 degrees (overlap corrected forindialite), and quartz at 20.8 degrees.

Example 1

This example illustrates the fabrication of a ceramic filter accordingto the current invention, wherein the membrane layer is applied to theouter surface of the support and is formed using a woven glass fabric.

An alumina slurry was prepared by charging 7.0 liters of water and 20.0ml of formic acid in a mixing vessel. Fumed alumina having an averageparticle size of 13-15 nm (manufactured and sold by Degussa Corp.,Ridgefield, N.J.) (2.0 kg) was added slowly with stirring. The pH of thedispersion was adjusted to 4.0 to 4.1 using formic acid. Afterstabilizing at this pH for two hours, 11.0 kg of Grade A-17 alumina(average particle size 23 microns, manufactured and sold by AlcoaIndustrial Chemicals Div., Bauxite, Ark.) was added in portions andstirred overnight. Glycerol was added to the slurry at a level of 3 wt %based on the total weight of the slurry. The solids content of thedispersion was 62-65 weight percent and the viscosity was adjusted to140 centipoise by water addition, measured with a Brookfield viscometer(Model No. RV1) using the #1 spindle.

A 2-ply glass yarn (150 filaments/ply) comprising 65.2% SiO₂, 23.8% Al₂O₃, and 10.0% MgO having a hydrophilic sizing to aid wetting by theaqueous coating composition (S glass, designation S-2 CG150 1/2 636,available from Owens-Corning Fiberglass Corporation) was fed through aball tensioner, passed through the alumina slurry, and pulled outthrough a 0.017 in diameter(0.043 cm) sizing orifice to remove excessslurry. The sizing orifice controlled the amount of slurry applied tothe yarn so that, after drying, about 50-60% by weight of ceramic in thesupport was from the slurry and about 40-50% by weight was derived fromthe yarn. The wet yarn was then passed through a guide attached to thetraverse arm of the filament winding machine and wound onto thecontoured mandrel described above wrapped with 2 layers of 0.002 in(0.005 cm) "Mylar" polyester film. The winding was stopped after about1000 grams of yarn were wound onto the mandrel, when the support reachedthe desired outside diameter (approximately 60 mm). After dryingovernight at room temperature, the filament-wound tube was removed fromthe mandrel by cutting through the wound material at about the center ofthe raised flange section (indicating the location of the plastic collarinsert) and removing the two pieces from the opposite ends of themandrel.

The outer membrane layer was attached to the support as follows. S-2glass fabric (plain weave, 1.5 oz/square yard) available from BurlingtonGlass Fabric (Altavista, Va.) was cut into pieces of length and widthapproximately equal to the length and circumference of the tuberespectively. Each piece was wrapped on the body of the tube and analumina slurry containing Grade A-16 alumina (manufactured and sold byAlcoa, average particle size 0.45 micron) with 55 to 60 weight percentsolid content, 3 wt % glycerol, and 100 to 120 cps viscosity, wasbrushed on the fabric. The fabric was not applied to the flange and thebottom end of the tube. Any wrinkles in the fabric were removed byrubbing with a wet sponge while the fabric was still wet before addingadditional layers of fabric. Two additional layers of fabric wereattached in a similar manner such that the closing of the ends in eachlayer of fabric fell approximately 120 degrees apart in the finalfilter. After all fabric layers were applied, the tube was driedovernight at room temperature. It was then low-temperature fired at 700°C. for one hour in a muffle furnace to remove volatiles and stabilizethe structure.

The flange section was reinforced and sealed by dipping one time in analumina slurry (fumed alumina/A-17 alumina, described above) anddraining off the excess. A wad of S-2 glass fibers was inserted into thehole in the bottom end of the filter and the bottom end was then dippedin the A-17 alumina slurry. After thorough drying and firing at 700° C.for one hour, the filter was fired in a high temperature furnace. Thetemperature was increased to 800° C. in about 40 minutes, held for about20 minutes, then increased to 1300° C. at a rate of 2° C./minute, heldfor 2 hours, then heated at a rate of 1° C./minute to 1380° C., held fortwo hours and cooled to 800° C. at a rate of 5° C./minute, followed byunrestrained cooling of the furnace to 200° C. The filter was thenremoved from the furnace and allowed to cool to room temperature in air.

The membrane layer had a bulk density of 1.62 g/cc and a volume porosityof 39% with a median pore diameter of 0.45 micron, measured by mercuryporosimetry. The average oxide composition of the filter, determined byX-ray fluorescence, was 27% silica, 68% alumina and 4% magnesia. Thecrystalline phase composition, determined by X-ray diffraction, was 35%cordierite (indialite), 6% mullite, 50% corundum and 9% cristobalite.

Example 2

This example illustrates the fabrication of a ceramic filter of thecurrent invention, wherein the outer membrane layer is formed bycircular winding.

A filter support was prepared in a manner similar to that described inExample 1 except that the filament-wound collar insert was used to formthe flange section instead of the plastic collar. When the supportelement was cut through for removal from the mandrel, the wound collarwas cut through as well such that a section of the original collarremained in the flange section of the support. The mandrel with thesupport wound thereon was immediately transferred to a specializedwinder for formation of the membrane layer.

The outer membrane was applied to the support by circular (hoop) windingof a glass yarn (Owens-Corning S-2 CG 150 1/2 636) on the surface of thesupport. The filament winder used for formation of the membrane layerhad a screw driven traverse, with the drive ratio set such that thespindle rotated at a speed of 75 complete revolutions for each 1 inch(2.54 cm) travel of the traverse guide so that the yarn was placed at aspacing of 75 yarns per linear inch (30 yarns per linear cm) of tubesurface. Adjacent yarn windings were as close to each other as possiblewithout overlapping. The yarn was soaked in the A-17/fumed aluminaslurry, and pulled through a 0.017 in (0.043 cm) sizing orifice prior towinding. About 60 grams of yarn were wound on the support surface toform a single layer of winding over its length. The circular winding wasdone across the entire length of the filter, bottom end and flangesection. After overnight drying (12-16 hours) at ambient temperature,the tube was removed from the mandrel as described in Example 1. Afterinspection for defects, the filter unit was fired at 700 degrees C. fortwo hours. Then the bottom hole was then filled with a wad of S-glassyarn. The flange and bottom sections of the tube were dipped in theA-17/fumed alumina slurry, the excess drained off, and dried thoroughly.The combined support and membrane was then high-temperature fired asdescribed in Example 1.

The membrane layer had a bulk density of 1.61 g/cc and a volume porosityof 39% with a median pore diameter of 0.43 μm, as measured by mercuryporosimetry. The average oxide composition of the filter, determined byX-ray fluorescence, was 27% silica, 68% alumina and 4% magnesia. Thecrystalline phase composition, determined by X-ray diffraction, was 33%cordierite, 8% mullite, 49% corundum and 10% cristobalite.

Example 3

This example illustrates the fabrication of a ceramic filter of thecurrent invention, wherein the outer membrane layer is formed bymultiple yarn winding.

A filter support element was prepared as described in Example 1.

The outer membrane layer was formed using the same filament winder aswas used to form the support. Yarns from three different bobbins of S-2CG 150 1/2 636 glass yarn were combined and fed through a tensiondevice, dipped in the A-17/fumed alumina slurry described in Example 1,pulled through a 0.025 in (0.64 mm) diameter sizing orifice, and woundon the support. The same wind angle, mandrel rotation rate, and traversearm speed used for the support was used for winding the membrane layer.The winding was continued until two layers of yarn had been wound ontothe mandrel so that the yarn covered the entire surface of the support.After drying overnight, the bottom end and flange sections were treatedas described in Example 2. The assembly was then high temperature firedas described in Example 1.

The membrane layer had a bulk density of 1.75 g/cc and a volume porosityof 37% with a median pore diameter of 0.64 μm, as measured by mercuryporosimetry. The average oxide composition, determined by X-rayfluorescence, was 27% silica, 68% alumina and 4% magnesia. Thecrystalline phase composition, determined by X-ray diffraction, was 35%cordierite, 6% mullite, 50% corundum and 9% cristobalite.

Example 4

This example illustrates the fabrication of a ceramic filter of thecurrent invention, wherein membrane layers are applied to both the innerand outer surfaces of the support and are formed by circular winding.

The inner membrane was formed by the circular winding of glass yarn,saturated with a ceramic particulate slurry, around a plastic-wrappedmandrel. After preparing a mandrel (as described in Example 1), afilament-wound collar insert (as described in Example 2) was positionedso as to leave about 57 inches (145 cm) of the straight tube portion ofthe mandrel exposed between the insert edge and the junction of the tubewith the hemispherical end closure. The mandrel was then placed on thesame circular (hoop) winder used in Example 2, but with the spindle setto rotate at a speed of about 71.6 complete revolutions for each 1 inch(2.54 cm) travel of the traverse guide so that the yarn was placed at aspacing of about 71.6 yarns per linear inch (28 yarns per cm) of tubesurface. The Owens-Corning "S-2 CG150 1/2 636" glass yarn was soaked inthe A-17/fumed alumina slurry, and pulled through a 0.015 in (0.038 cm)diameter sizing orifice prior to winding. Using approximately 60 gramsof yarn, the mandrel was wrapped from the junction of the tube with thehemispherical end closure to the edge of the filament-wound collarinsert.

While the yarn was still wet, a filter support was laid down,essentially as described in Example 1, using the chain-driven filamentwinder. The support was wound over the hemispherical end, the innermembrane and the filament-wound collar insert.

When the support was complete it was put back on the circular (hoop)winder and the flange and tip ends of the unit were reinforced asfollows by the infiltration of alumina slurry. While the mandrel wasrotated at 100 RPM, approximately 20 cc of the A-17/fumed alumina slurrydescribed in Example 1 was slowly poured onto and absorbed into about 2inches (5.1 cm) of the tip of the unit. An additional 20 cc of slurrywas similarly applied to the flange region, from the shoulder of theflange to 1.5 inches (3.7 cm) below the collar insert. When the slurryhad been absorbed into the tip and flange by capillary action, and therewas no excess slurry on the surface, the outer membrane was hoop woundonto the support layer, essentially as described in Example 2. Duringeach of the winding procedures, a humidity level of at least 30% wasmaintained.

Heat guns were set up to dry the infiltrated regions, while maintainingrotation, for at least 20 minutes. Afterward, the mandrel was removedfrom the winder and placed in a vertical support rack.

After overnight drying (about 12-16 hours) at ambient temperature (e.g.,about 20° C.), the tube comprising an inner membrane, a support layerand an outer membrane, was removed from the mandrel as described inExample 1.

After inspection for defects, the open hole in the tip of the unit wasfilled with a paste comprising by weight about 68% of the A-17/fumedalumina slurry described in Example 1, about 3% short staple S-glassyarn, and about 29% partially-fired alumina-coated S-glass particulate(e.g., S-glass yarn coated with matrix slurry, as described above, thenheated to approximately 700° C. to partially crystallize the glass yarn,then comminuted).

The filter was fired to a peak temperature of 1380° C. following thecycle described in Example 1.

Example 5

A ceramic filter was prepared in substantially the same manner as thefilter described in Example 4, with the following notable exceptions:

No inner membrane layer was produced.

After winding the outer membrane (as shown in FIG. 3A) and firing toabout 700° C. to stabilize the structure, as described in Example 1, themembrane was coated with an aqueous slurry comprising equal weightfractions of 400 grit 38 Alundum® alumina particulate (23 microns ave.particle size, Norton-St.Gobain, Worcester, Mass.) and Bluonic®colloidal alumina (obtained from Wesbond Corp., Wilmington, Del.). Theslurry was applied by brush to the outer surface of the filament woundsupport. The liquid component of the slurry was quickly absorbed by thesupport, leaving a buildup of particulates on the surface. The excessalumina particulate was manually removed by gently rubbing the surface;then the developing filter was dried overnight at room temperature. Thedeveloping filter was then again low-temperature fired at about 700° C.for one hour in a muffle furnace to remove volatiles. The filteringsurface created by this addition of filler material such as particulateon top of a "hoop membrane" is also considered to be a "combinationmembrane".

Example 6

This Example demonstrates, among other things, the fabrication of afilament wound ceramic hot gas filter featuring a "combination" typemembrane.

First, a slurry for coating yarns was prepared by charging a mixingvessel with about 90 kg of "Chorhydrol® 50%" aluminum chlorhydratesolution (Reheis, Inc., Berkeley Heights, N.J.). While stirring, about113 kg of Grade A-17 alumina powder (2-3 microns ave. particle size,Alcoa Industrial Chemicals Div., Bauxite, Ark.) and about 1435 g ofhydrochloric acid were added to the solution.

Next, a filament wound collar insert (described previously) was insertedon the mandrel near the 57 inch (145 cm) position (described previously)to later form the flange section of the filter tube.

A 2-ply glass yarn (150 filaments/ply) comprising 65.2% SiO₂, 23.8% Al₂O₃, and 10.0% MgO having a hydrophilic sizing to aid wetting by theaqueous coating composition (S glass, designation S-2 CG150 1/2 636,available from Owens-Corning Fiberglass Corporation) was fed through aball tensioner, passed through the alumina slurry, and pulled outthrough a 0.017 in diameter(0.043 cm) sizing orifice to remove excessslurry. The sizing orifice controlled the amount of slurry applied tothe yarn so that, after drying, about 50-60% by weight of ceramic in thesupport was from the slurry and about 40-50% by weight was derived fromthe yarn. The wet yarn was then passed through a guide attached to thetraverse arm of the filament winding machine and wound onto thecontoured mandrel described above wrapped with 2 layers of 0.002 in(0.005 cm) "Mylar" polyester film. The winding was stopped after about1000 grams of yarn were wound onto the mandrel, when the support reachedthe desired outside diameter (approximately 60 mm).

The mandrel with the support and collar insert thereon was thentransferred to a specialized winder. The flange end of the developingfilter unit was then reinforced as follows. With the mandrel rotating atabout 100 RPM, approximately 20 cc of the above-described alumina slurryused for yarn coating was slowly poured onto the flange region, from theshoulder of the flange to about 1.5 inches (3.7 cm) below the collarinsert. When the slurry had been absorbed into the flange by capillaryaction, and there was no excess slurry on the surface, construction ofthe membrane layer commenced.

The yarn portion of the membrane layer was applied by hoop winding.Specifically, the filament winder used for formation of the membranelayer had a screw driven traverse, with the drive ratio set such thatthe spindle rotated at a speed of about 46.8 complete revolutions foreach 1 inch (2.54 cm) travel of the traverse guide so that the yarn wasplaced at a spacing of 46.8 yarns per linear inch (18.4 yarns per linearcm) of tube surface. This spacing is such as to leave a gap betweenwindings roughly equal to the width of the yarn. The yarn was soaked inthe A-17/Chlorhydrol® alumina slurry, and pulled through a 0.017 inch(0.043 cm) sizing orifice prior to winding. The circular winding wasdone across the entire length of the filter, bottom end and flangesection. During winding, a humidity level of at least 30 percent wasmaintained.

Heat guns were set up to dry the reinforced regions, while maintainingrotation, for at least 20 minutes. The developing filter and mandrelwere then removed from the winder and placed in a vertical support rack.

After overnight drying (about 12-16 hours) at ambient temperature, theends of the developing filter were cut off so that the mandrel could beremoved. The collar portion of the filter was sliced such that a sectionof the original collar remained in the flange section of the supportlayer. The developing filter was then heated from ambient to atemperature of about 700° C. in a muffle furnace equipped with ahydrochloric acid scrubber. After maintaining this low firingtemperature for about one hour, the furnace and its contents werepermitted to furnace cool.

Next, a paste for closing the tip end of the tube and for filling in thegap between windings in the membrane layer was prepared. Specifically,about 980 g of de-ionized water was measured out in an open container.While stirring, about 20 g of "Superloid" ammonium alginate (Kelco Co.,San Diego, Calif.) was added. Stirring of this mixture was continueduntil a smooth-flowing solution, free from gel particles, was obtained.Then, while continuing to stir, about 330 g of talc (Grade MP 12-62,manufactured by Minerals Technologies) was added to the solution. Whenthe talc had been evenly dispersed, an additional 2700 g of 320 grit 38Alundum® alumina particulate (Norton-St. Gobain, Worcester, Mass., 32microns ave. particle size) was slowly added. Mixing was continued untila smooth paste, without apparent lumps or agglommerates, was obtained.

The low fired candle filter was slid back onto a mandrel and put backonto the winder. The mandrel was rotated at approximately 100 RPM whilethe particulate paste was applied to the surface of the filter with aplastic spatula until the entire surface was covered. Sufficientpressure and "drag" were then applied with a clean spatula to removemost of the excess material. A cross-sectional schematic view of thetube wall is illustrated in FIG. 3B.

After removing the developing candle filter from the mandrel once again,the 1/4 inch (6 mm) diameter opening in the tip of the candle was filledwith the above-identified paste. After overnight drying, a 1.25 inch (32mm) diameter, 4 inch (102 mm) long, 40-watt illuminated light bulb wasinserted into the open end of the filter. All room lights wereextinguished and the surface of the filter was examined. In any locationwhere there were bright points of light ("pin holes") additionalparticulate paste was applied.

The candle filter was then high temperature fired as follows. The candlefilter was placed into an air atmosphere furnace at about ambient (e.g.,about 20° C.) temperature. The furnace temperature was increased toabout 800° C. in about 40 minutes, held for about 1 hour, then increasedto about 1300° C. at a rate of about 20 per minute, held for about 2hours, then increased to about 1380° C. at a rate of about 1° C. perminute, held for about 2 hours, cooled to about 800° C. at a rate ofabout 5° C. per minute, and finally furnace cooled to about 200° C. Thefurnace was then opened and its contents permitted to cool naturally toambient temperature.

Example 7

A ceramic hot gas filter was produced substantially in accordance withExample 5 except that no circularly wound filaments or yarns made up themembrane layer. The slurry for the membrane layer was applied by brush,and excess particulates were gently rubbed off of the filter tube. Across-sectional schematic view of the tube wall is illustrated in FIG.3C.

What is claimed is:
 1. A ceramic hot gas filter, comprising:a porouselongated filter support, said support having an outer surface, anopening at one end into a hollow interior defined in part by an innersurface, a closed end opposite said open end, and an external flangeintegral with said open end, said support being formed of a plurality oflayers of oxide ceramic yarn, each layer being arranged in acrisscrossing relationship with neighboring layers to form a pluralityof quadrilateral-shaped openings, said yarn being coated with firstoxide ceramic material, said first oxide ceramic material providing,upon heat treatment, a porous refractory oxide support matrix; and aporous membrane layer contacting the outer surface or inner surface ofsaid support, said membrane layer being less porous than said supportand comprising (1) at least one circularly wound continuous filamentoxide ceramic yarn, adjacent windings of said ceramic yarn defining agap therebetween, said yarn being coated with a second oxide ceramicmaterial, and (2) at least one ceramic filler material disposed in saidgap and substantially uniformly distributed therein.
 2. The filter ofclaim 1, wherein said at least one ceramic filler material comprises aparticulate oxide ceramic material.
 3. The filter of claim 1, whereinsaid at least one ceramic filler material comprises a form selected fromthe group consisting of powders, particulates, whiskers, chopped fibers,platelets, flakes, spheres, tubules and pellets.
 4. The filter of claim1, wherein said filter has a crystalline composition of about 25-40%cordierite, 5-15% mullite, 40-60% corundum, and 0-10% cristobalite,based on the total crystalline content of the filter.
 5. The filter ofclaim 1, wherein said quadrilateral-shaped openings have dimensions ofabout 100 to about 500 microns after heat treatment.
 6. The filter ofclaim 1, wherein said membrane layer defines pores having diameters ofabout 0.1 to about 50 microns.
 7. The filter of claim 1, wherein saidmembrane layer defines pores having diameters of about 5 to about 25microns.
 8. The filter of claim 7, wherein an average size and sizedistribution of said pores is substantially invariant around acircumference and along a longitudinal extent of said membrane layer. 9.The filter of claim 1, wherein said second oxide ceramic materialprovides, upon heat treatment, a porous refractory oxide membranematrix.
 10. The filter of claim 9, wherein between about 40 to about 70percent of the total weight of said filter is from the combined weightof said support matrix and said membrane matrix.
 11. The filter of claim1, wherein said support yarn has generally the same composition as saidcontinuous filament membrane yarn and wherein said first oxide ceramicmaterial has generally the same composition as said second oxide ceramicmaterial.
 12. The filter of claim 1, wherein said first and second oxideceramic materials each comprise Al₂ O₃.
 13. The filter of claim 1,wherein said porous membrane layer contacts both the outer surface andthe inner surface of said porous elongated filter support.
 14. A ceramichot gas filter, comprising:(a) a porous elongated filter support, saidsupport having an outer surface, an opening at one end into a hollowinterior defined in part by an inner surface, and a closed end oppositesaid open end, said support being formed of a plurality of layers ofoxide ceramic yarn, each layer being arranged in a crisscrossingrelationship with neighboring layers to form a plurality ofquadrilateral-shaped openings, said yarn being coated with first oxideceramic material, said first oxide ceramic material providing, upon heattreatment, a porous refractory oxide support matrix; and (b) a porousmembrane layer contacting at least one of the outer surface or innersurface of said support, said membrane layer being less porous than saidsupport and comprising (1) at least one hoop wound continuous filamentoxide ceramic yarn, adjacent windings of said ceramic yarn defining agap therebetween, said yarn being coated with a second oxide ceramicmaterial, said second oxide ceramic material providing, upon heattreatment, a porous refractory oxide membrane matrix, and (2) at leastone ceramic filler material deposited in said gap and substantiallyuniformly distributed therein.
 15. The filter of claim 14, wherein saidsupport yarn and said continuous filament membrane yarn each comprise atleast 20 weight percent alumina and have a softening point above about750° C.
 16. The filter of claim 14, wherein a weight ratio of said yarnto said matrix is essentially the same in said membrane layer as in saidsupport.
 17. The filter of claim 3, wherein said first and said secondoxide ceramic materials comprise particulate, and further wherein saidform of said at least one ceramic filler material has a size which islarger than a size of said particulate.
 18. The filter of claim 14,wherein said at least one filler material comprises bodies having a sizeof about 25 to 75 microns.
 19. The filter of claim 14, wherein saidmembrane layer contacts the outer surface of said support, and is madeby a method comprising:(a) fabricating said elongated porous filtersupport by coating a ceramic oxide support yarn with a first coatingcomposition, winding said coated ceramic oxide support yarn onto amandrel to form a plurality of layers of said coated support yarn, eachlayer being arranged in a crisscrossing relationship with neighboringlayers to form a plurality of quadrilateral-shaped openings, said firstcoating composition providing, upon heat treatment, a porous refractoryoxide support matrix; (b) coating at least one continuous filament oxidemembrane yarn with a second coating composition, and winding said coatedyarn onto said filter support, said winding being conducted so as toleave a gap between adjacent windings of said coated yarn; (c)depositing a slurry or paste comprising a suspending agent and at leastone ceramic filler material into said gap; (d) drying said paste to formsaid membrane layer; and (e) firing said support and membrane layers.